This invention relates to a security element for electronic article surveillance, comprising one lower and one upper conducting track having at least one winding each, the two conducting tracks being wound in opposing directions and overlapping in at least one area of over-lap with a dielectric layer provided in the area of overlap between the two conducting tracks.
A corresponding security element is known from German Patent DE 197 08 180 A1, for example. Security elements of this type are referred to as resonant circuits or resonant frequency security elements, the resonant frequency being determined by the capacitance C, the inductance L and the resistance R of the resonant circuit. Resonant frequency security elements in the form of labels or tags are used in the prevention and detection of theft in department stores and warehouses, and they are becoming increasingly popular as a result of their high rates of detection.
Article surveillance itself can be described briefly as follows: The resonant frequency security elements are excited by an alternating magnetic field in the interrogation zone of the area to be maintained under surveillancexe2x80x94this is conventionally the entrance and exit area of a department store or warehousexe2x80x94so that they emit a detectable characteristic signal. Once this characteristic signal is detected by the surveillance system, an alarm is produced.
It is particularly advantageous to construct the resonant frequency security elements so that they can be deactivated. Deactivation takes place as soon as the protected article has been rightly purchased by a customer. One method that has proven successful in this connection is to produce a short-circuit between the two layers of conducting tracks by applying an accordingly high pulse of energy through the dielectric layer, thus leading to the destruction of the circuit""s typical resonant characteristics.
A deactivatable security element and a suitable production method therefor are already known from European Patent EP 0 665 705 A2. In the this Patent, each of the conducting track layers is comprised of a plurality of turns. The two layers of conducting tracks are interconnected by a dielectric, very thin layer of resin. This resin layer has an essentially constant thickness over the entire area of the layers.
Deactivation of the security element takes place in this case, too, by applying a sufficiently high pulse of energy.
Although the above described resonant circuit can be deactivated easily and reliably, there is a risk of it being reactivated by the application of mechanical strain, particularly bending or twistingxe2x80x94in other words, short-circuiting can be canceled again by the mechanical strain. Needless to say, reactivation is a highly undesirable effect.
To counter the risk of a once deactivated resonant frequency security element being reactivated accidentally, it is proposed in the previously mentioned German Patent DE 197 08 180 A1 making the strength of the two overlapping conducting tracks so great as to cause the security element to bend, if subjected to mechanical strain, in those areas which are essentially free of conducting tracks (xe2x86x92zones of preferred bending). Thanks to this construction the probability of reactivating the security element can be appreciably reduced, because reactivation takes place only when the point of short-circuiting lies in the zones of preferred bending. Since these zones make up only a relatively small part of the overall area of the overlapping conducting tracks, the risk of reactivating a security element is naturally reduced to a considerable degree.
Further investigations have shown that areas in which one of the two conducting tracks crosses end areas of the other conducting track (critical area) are more susceptible to reactivation than the remaining areas of overlap. In other words: an electrical connection between the two conducting tracksxe2x80x94hence with deactivating effect on the resonant circuitxe2x80x94caused by bending or twisting of the security element is more likely to be broken when it lies in the critical area of overlap than when it occurs in the remaining area of overlap between the conducting tracks.
It is an object of the present invention to provide a security element in which the risk of reactivation after deactivation is reduced or fully eliminated.
This object is accomplished with respect to the security element of the present invention in that in a critical area in which the upper and lower conducting tracks overlap a border edge of the lower and upper conducting tracks, respectively, the dielectric layer is at least as thick as, or thicker than, in the remaining area of overlap. According to a preferred embodiment of the security element of the present invention, the dielectric layer is a dielectric adhesive layer.
As previously explained, areas in which a border edge of a conducting track is crossed by the other conducting track (critical area) are particularly susceptible to reactivation when the preceding deactivation took place in exactly these areas. It is impossible, by the way, to prevent such areas occurring in cases where two conducting tracks are wound in opposing directions.
The security element has turned out to be particularly resistant to reactivation when the sum of the lengths of the border edges is minimal in the critical area or in the critical areas or in the strip-shaped area.
In a further advantageous aspect of the security element of the present invention, it is thus proposed to provide as the additional dielectric layer a dielectric film material, a dielectric lacquer layer or an additional dielectric adhesive layer in the critical area or critical areas in addition to the dielectric adhesive layer. Alternatively, it is proposed to achieve greater thickness in the critical area or critical areas by subjecting the two conducting tracks to a lower application pressure in these areas during the lamination process.
In the event of there being several areas in which a border edge of the upper and lower conducting tracks overlap with the lower and upper conducting tracks, respectively, an advantageous embodiment of the security element of the present invention provides for these areas to be arranged essentially along a straight line in an essentially strip-shaped area. This strip-shaped area is a few millimeters wide and extends preferably over the fill length of the security element. It is an advantage of course if the width of the additional dielectric layer is coordinated with the width of the strip-shaped area.
According to a preferred embodiment of the security element of the present invention, the two conducting tracks are connected electrically in an overlapping end area. The electrical connection can be effected in various ways. Alternative possibilities are described in German Patent DE 197 08 180. The preferred method for bonding the two conducting tracks involves, in conjunction with the present invention, perforating the security element in the area of the conducting tracks in which the electrical connection is desired.
A particularly favorable method of manufacturing the security elements of the present invention involves the following process steps: A dielectric adhesive layer is applied to the lower conducting track; the critical area or the critical areas is or are coated with an additional dielectric layer which may be a dielectric film material, a dielectric lacquer layer or an additional dielectric adhesive layer; the upper conducting track is then joined to the lower conducting track in the desired position. Alternatively, provision is made for the lower and upper conducting tracks to be laminated together in the critical area or critical areas using a reduced application pressure. The laminating the two conducting tracks at a reduced application pressure can also be performed in combination with the arrangement of an additional dielectric layer in the critical area or critical areas.
In the event of mechanical production, an advantageous further aspect of the method of manufacturing the security element of the present invention provides for the upper and lower conducting tracks to be positioned each on a web material; the conducting tracks are arranged on the web material so that the critical areas are oriented in the running direction of the machine; the additional dielectric layer is applied in strip form to the lower or upper conducting track, as applicable, before the two conducting tracks are laminated together.
An advantageous embodiment of the method for manufacturing the security elements of the present invention provides for the additional dielectric layer to be applied to the upper or lower conducting track as a dielectric lacquer layer by spraying or printing. If an additional dielectric adhesive layer is involved, it is preferably applied to the upper or lower conducting track by means of extrusion.
The present invention will be explained in more detail in the following with reference to the accompanying drawing.